1. Field of the Invention
The present invention is directed to an electrolyte for very high voltage electrolytic capacitors and to an electrolytic capacitor impregnated with the electrolyte of the present invention for use in an implantable cardioverter defibrillator (ICD).
2. Related Art
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an Implantable Cardioverter Defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Implantable Cardioverter Defibrillators, such as those disclosed in U.S. Pat. No. 5,131,388, incorporated herein by reference, typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Conventionally, such electrolytic capacitors include an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. While aluminum is the preferred metal for the anode plates, other metals such as tantalum, magnesium, titanium, niobium, zirconium and zinc may be used. A typical solvent-based liquid electrolyte may be a mixture of a weak acid and a salt of a weak acid, preferably a salt of the weak acid employed, in a polyhydroxy alcohol solvent. The electrolytic or ion-producing component of the electrolyte is the salt that is dissolved in the solvent. The entire laminate is rolled up into the form of a substantially cylindrical body, or wound roll, that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, comprising a planar, layered, stack structure of electrode materials with separators interposed therebetween, such as those disclosed in the above-mentioned U.S. Pat. No. 5,131,388.
In ICDs, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. Since the capacitance of an electrolytic capacitor is provided by the anodes, a clear strategy for increasing the energy density in the capacitor is to minimize the volume taken up by paper and cathode and maximize the number of anodes. A multiple anode stack configuration requires fewer cathodes and paper spacers than a single anode configuration and thus reduces the size of the device. A multiple anode stack consists of a number of units consisting of a cathode, a paper spacer, two or more anodes, a paper spacer and a cathode, with neighboring units sharing the cathode between them. Energy storage density can be increased by using a multiple anode element, however, the drawback is that the equivalent series resistance, ESR, of the capacitor increases as the conduction path from cathode to anode becomes increasingly tortuous.
Typically, an implantable cardioverter defibrillator may utilize two 350 to 400 volt electrolytic capacitors in series to achieve a voltage of 700 to 800 volts. However, for greater ease of construction and a thinner ICD, a single capacitor unit is preferred. Accordingly, the need for a capacitor with a working voltage of greater than 400 volts becomes pronounced. Commercial capacitors rated to 600 volts can be obtained but generally suffer from high equivalent series resistance (ESR) due to low electrolyte conductivity. In addition, capacitors with voltage ratings in excess of 700 volts are not available.
There are numerous commercially available compositions of electrolyte for use in electrolytic capacitors that can conform to reasonable specifications, as long as the operating voltage of the capacitor remains at 400 volts or lower. However, once this limit is exceeded, the choices become more limited. There are relatively few electrolytes for this voltage regime, and the suitable electrolytes known in the art have several drawbacks, especially when used in a flat, stacked capacitor having a multiple anode configuration. First, glycol-based electrolytes suffer from relatively poor conductivity and ionic mobility. These electrolytes will produce a capacitor with significant energy loss due to a higher than acceptable equivalent series resistance (ESR). Second, γ-butyrolactone based electrolytes, which overcome the problems of ionic mobility, can not be used in conjunction with typical paper spacer pads. These require thicker, more expensive pads made out of manila fibers, and as a result of greater thickness, sharply reduce the energy density in flat stacked capacitor designs. Many high voltage electrolytes employ the use of very long chain dicarboxylic acids and large bases to achieve the necessary breakdown voltages, however, the resultant electrolytes have very low conductivities (≦1 mS/cm). For example, U.S. Pat. No. 4,860,169 discloses an electrolytic capacitor for use in operation at voltages above 500 volts, produced by employing an electrolyte containing a straight chain saturated aliphatic dicarboxylic acid in which the carboxylic moieties are separated by at least 14 carbon atoms in a mixture of at least one polar organic solvent and water. The disclosed composition has a resistivity at 30° C. of 1280 Ω·cm (781 μS/cm), a pH of 9.68 and Scintillation voltage of 500 V.
What is needed in the art is an electrolyte that provides acceptable breakdown characteristics with reasonable conductivity when impregnated in an electrolytic capacitor operating above 700 volts.